Unique silica polymorph obtained under electron irradiation

High purity synthetic silica glass (Suprasil F300) samples were densified by High Pressure/High Temperature (HP-HT) using three different pressure/temperature/duration values. Their relaxation process was studied by applying 2.5 MeV electron irradiation with doses varying from 1 MGy up to 11 GGy. At very high dose (11 GGy), all the densified silica samples exhibit the same density value (∼2.26 g/cm³) regardless of the densification conditions, referring to an equilibrium value known as an amorphous silica metamict phase. In detail, the HP-HT silica samples were progressively dedensified from 1 GGy irradiation dose until reaching this equilibrium. Whatever the initial topological disorder revealed through Raman spectra, all silica samples present identical Raman spectra at 11 GGy, with high intensities of D₁ and D₂ components, attesting they reach one unique silica polymorph. Regarding the irradiation at high temperature (600 MGy, 600 K), the results show that the thermal annealing during irradiation "accelerates" somehow the relaxation kinetics of both macroscopic density and vibrational signature. Furthermore, it seems that such high irradiation temperatures lead to increase the Non-Bridging Oxygen Hole Center point defects generation.